Capacitors are widely used in integrated circuit devices, such as Dynamic Random Access Memory (DRAM) devices. As DRAM devices become more highly integrated, various approaches for increasing the capacitance within a defined cell area have been proposed. For example, one approach is to thin the dielectric film. A second approach is to make the capacitor three-dimensional to increase the effective area thereof. A third approach is to use a material having a high dielectric constant. These approaches can also be combined.
Unfortunately, in the first approach, when the thickness of the dielectric film is 100 .ANG. or less, the reliability may deteriorate due to Fowler-Nordheim currents. This may limit the ability to fabricate ultra-thin dielectric films.
In the second approach, complicated processes and high product cost may result when fabricating a three-dimensional capacitor, such as a cylindrical type or a pin type capacitor. This may limit the advantages of three-dimensional capacitors.
For the third approach, various proposals for increasing capacitance using a high dielectric contact material have been made. For the high dielectric constant material, ferroelectric materials such as SrTiO.sub.3 Ba(Sr, Ti)O.sub.3 (BST), Pb(Zr, Ti)O.sub.3 (PZT), Pb(La, Zr)TiO.sub.3 and Ta.sub.2 O.sub.5 may be used.
A metal of the platinum group or an oxide thereof has been used for an electrode material of the high dielectric capacitor. It will be understood that the platinum group metals include the following six metals, all of which are members of Group VIII of the periodic system: ruthenium, rhodium, palladium, osmium, iridium and platinum. The platinum group metals have an excellent oxidation resistance, so they are not oxidized even where they contact the high dielectric layer. Further, platinum has an excellent leakage current characteristic. That is, platinum (Pt) has a work function higher than that of high dielectric material such as BST, SrTiO.sub.3 and PZT, so that a Schottky barrier is formed at an interface with the high dielectric material, which can provide excellent leakage current characteristics.
However, platinum may react with polysilicon at a temperature of 300.degree. C. or higher, to thereby form silicide. Accordingly, it is known to form a barrier layer for preventing silicide formation, at a lower portion of a platinum electrode. Unfortunately, materials commonly used for the barrier layer, for example metal nitrides such as TiN, TaN or WN.sub.1-x, may be oxidized during a subsequent heat treatment in an oxygen atmosphere.
In particular, the platinum electrode formation generally is performed at a high temperature so that the surface of the electrode is smooth. Subsequently, a high dielectric constant layer is deposited in an oxygen atmosphere at a high temperature. Unfortunately, oxygen may flow into the barrier layer along a grain boundary of the platinum electrode, and then it may additionally diffuse during the subsequent heat treatment. As a result, nitrogen contained in the barrier layer may be replaced with oxygen, which may cause the barrier layer to peel off.
In order to overcome the above problem, it has been proposed to use a conductive oxide layer such as RuO.sub.2 or IrO.sub.2 for the capacitor electrode. In FIG. 1, a high dielectric constant capacitor having this structure is shown.
Referring to FIG. 1, reference numerals 10, 11 and 12 indicate an integrated circuit substrate such as a semiconductor substrate, an insulating layer and a conductive plug, respectively. Reference numerals 13, 14 and 15 indicate an ohmic layer, a barrier layer and a lower electrode, respectively. Reference numerals 17 and 19 indicate a high dielectric constant layer and an upper electrode, respectively.
In FIG. 1, the conductive plug 12 is formed of polysilicon, and the barrier layer 14 is formed of a metal nitride such as TiN. Also, the lower electrode 15 is formed of a conductive oxide layer, for example, RuO.sub.2. The high dielectric constant layer 17 can be formed of a high dielectric constant material such as BST, PZT, PLZT or Ta.sub.2 O.sub.5, and the upper electrode 19 can be formed of a metal of the platinum group or an oxide thereof.
Unfortunately, although the conductive oxide layer forming the lower electrode 15 can prevent oxygen diffusion more effectively than platinum, it has a work function similar to that of the high dielectric constant layer, which can cause inferior leakage current characteristics. In order to improve the leakage current characteristics, the contact area between the lower electrode 15 and the high dielectric constant layer 17 can be reduced, for example, by reducing the thickness of the lower electrode 15. However, there may be a limit as to how thin the lower electrode 15 can be made. This is because the lower electrode 15 also functions as an oxygen diffusion barrier layer.
Accordingly, it has also been proposed to thin the lower electrode by forming a sacrificial layer comprising a platinum group metal, between the lower electrode layer and the barrier layer. The sacrificial layer can prevent oxidation as will now be described in connection with FIG. 2.
FIG. 2 shows a high dielectric constant capacitor including a sacrificial layer. Referring to FIG. 2, reference numerals 200, 201 and 203 indicate an integrated circuit substrate such as a semiconductor substrate, an insulating layer and a conductive plug, respectively. Reference numeral 205 indicates an ohmic layer formed of TiSi.sub.x, and reference numeral 207 indicates a barrier layer formed of a TiN layer. Also, reference numerals 209 and 211 indicate a sacrificial layer for preventing oxidization and a lower electrode, respectively. Reference numerals 213 and 215 indicate a high dielectric constant layer and an upper electrode, respectively.
The sacrificial layer 209 for preventing oxidization can be formed of a platinum group metal, for example, Ru. Ru reacts with oxygen which flows into the Ru layer during a subsequent processing performed under an oxygen atmosphere at a high temperature, thereby forming RuO.sub.2. Accordingly, the sacrificial layer 209 for preventing oxidization blocks oxygen from flowing into the barrier layer 207, which can prevent oxidation of the barrier layer 207.
However, in the above-described high dielectric constant capacitor, the leakage current characteristics may still be inferior. Specifically, it is difficult to prevent oxygen from flowing from the sidewall of the barrier layer 207. Accordingly, oxidization of the barrier layer 207 may not be sufficiently prevented.